1. Field of the Invention
The present invention relates to a vehicular transmission system including a master friction clutch and, in particular, to a vehicular transmission system and method of operation to accommodate clutch failure.
2. Description of the Related Art
Centrifugally operated friction clutches are well known in the art and typically include a driving input member driven by a prime mover, usually an internal combustion engine, and weights rotatable with the driving member which, upon rotation of the driving member, will move radially outwardly under the effect of centrifugal force to cause the driving input member to frictionally engage a driven output member. Examples of centrifugally operated clutches may be seen by reference to U.S. Pat. Nos. 3,580,372; 3,580,372; 3,696,901; 5,437,356; 3,810,533; 4,819,779; 5,441,137; 5,730,269 and 4,610,343, the disclosures of which are incorporated herein by reference.
Vehicular transmission systems that employ a centrifugal master friction clutch for drivingly connecting an internal combustion engine to a multiple ratio transmission are also known in the art. When mated to an internal combustion engine, centrifugal clutches are engaged and disengage as a function of engine speed, requiring manipulation of engine speed to selectively engage and disengage the clutch. Vehicular transmission systems, especially for heavy-duty vehicles, may be configured to automatically shift the transmission from neutral into a low gear ratio, such as the starting gear, as the vehicle slows toward rest in what is commonly referred to as a “coast mode” of operation. To engage a low gear ratio during the “coast mode,” the engine speed must be slow enough to allow the centrifugal clutch to disengage.
However, failure of the clutch to disengage as the engine speed decreases permits the engine to transfer power through the clutch and into the transmission after the transmission is shifted from neutral into a low gear ratio. The clutch may fail to disengage for several reasons, including without limitation, damage of a clutch component that renders the centrifugal mechanism inoperable and/or failure of the engine speed to decrease below the speed that permits clutch disengagement. Under these circumstances, the vehicle operator may notice the engine begin to pull against the application of the vehicle service brakes as the vehicle is slowed. With increased brake application, the engine torque may disengage the clutch or stall the engine as the vehicle slows toward rest.
For these and other reasons, it is desirable to provide an improved approach for operating a vehicular transmission system, particularly those that employ a centrifugal master friction clutch, to reduce or minimize the known limitations of the prior art.